A magnetic field is a force field, invisibly pushing electrically charged objects just as a gravitational field pulls objects with mass. Whereas all objects with mass exert a gravitational field, however, not all objects have a magnetic field. Magnetic fields are created by electrical charges. Thanks to their structure at the atomic level, some substances -- like the iron in magnets -- have a permanent magnetic field.

Electrical Charges

A magnetic field is created by electrical charges in motion. All electrical currents create magnetic fields, since a current is electrons moving through a wire, and electrons all have a negative charge. Electrons in atoms also have magnetic fields because they are in constant motion. For atoms and molecules in which all the electrons are paired off, these magnetic fields cancel out, and the result is no net magnetic field. Substances in which atoms or molecules have unpaired electrons may be magnetic, however, and these are divided into two categories: paramagnetic and ferromagnetic.

Paramagnetic Materials

In a paramagnetic material, one or more electrons in each atom or molecule is unpaired and thus creates a net magnetic field for that atom or molecule, so each atom or molecule is like a tiny bar magnet. These magnetic fields, however, are all randomly aligned, and so overall they cancel each other out. That's why the paramagnetic material doesn't exhibit any of the same behavior as, say, the iron in a toy magnet -- it can't pick up iron filings, for example. But if the paramagnetic material is exposed to an external magnetic field like the one created by a coil of wire, the atoms or molecules tend to line up with this field and therefore experience a weak attraction toward it. So paramagnetic materials are magnetic in the sense that they are weakly attracted to external magnetic fields. Oxygen molecules are one example.

Ferromagnetic Materials

Ferromagnetic materials like iron also have atoms or molecules that contain electrons with unpaired spins. In these materials, however, all the atoms or molecules in small regions called domains are aligned with each other, although the domains themselves are all aligned randomly with respect to each other. Each domain is now a little like a tiny bar magnet. When the ferromagnetic material is exposed to an external magnetic field, the domains all line up and now the material is attracted to the magnet just as with a paramagnetic material. Unlike with paramagnetic materials, however, the domains now remain permanently aligned. The material is now permanently magnetized and creates a magnetic field that attracts other ferromagnetic or paramagnetic materials and objects. Iron bar magnets are perhaps the most common example.

Effects on Materials That Conduct Electricity

Since paramagnetic materials create no magnetic field of their own, their magnetic properties become apparent only when they are exposed to a field. They cannot induce electric currents. Ferromagnetic materials, by contrast, are surrounded by a permanent magnetic field once they have been magnetized. Since a changing magnetic field creates an electric current, putting a ferromagnetic material near a conductor like a loop of metal wire and then moving it creates a current in the wire that lasts only so long as the magnet keeps moving.

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About the Author

Based in San Diego, John Brennan has been writing about science and the environment since 2006. His articles have appeared in "Plenty," "San Diego Reader," "Santa Barbara Independent" and "East Bay Monthly." Brennan holds a Bachelor of Science in biology from the University of California, San Diego.